1. Field of the Invention
The present invention relates to an improved building method and in particular to improvements in design and construction of reinforced concrete slabs.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Concrete and steel reinforced slabs, beams, columns and walls are commonly used in all aspects of the building and construction industry. The applications include roadways, bridges, car parks, slabs in commercial, industrial and residential buildings which also incorporate concrete beams, columns and an assortment of reinforced concrete shapes and designs. The reinforced concrete structures come in many different arrangements and include forms called post-tensioned, composite, precast and conventional slabs on ground or in a suspended form employing frames and formwork which is erected on site.
The vast majority of reinforced concrete structures are poured in situ and generally have deformed and hi strength steel reinforcing bars designed into them. The reinforcing bars provide the tensile strength the structures needs to support itself and the construction and design loads applied to it on the completion of its installation. Structures such as composite slabs, including ones comprising composite metal decks and precast may also require the addition of reinforcement bar in the design.
Generally all reinforced concrete structures require steel reinforcement in many different arrangements and layers. In reinforced concrete slabs designs for example, multiple layers of reinforcement steel are laid in two directions at 90 degrees to each other. A lower layer placed adjacent the bottom of the slab to be formed provides tensile reinforcement to the structure. An upper layer laid adjacent the top of the slab to be formed assists with compression loadings and concrete shrinkage control. The steel mesh, or assembly of reinforcement bar adjacent the bottom of the slab provides tensile reinforcement and the upper layer provides negative reinforcement over the supports and is also used to resist shrinkage cracking of the slab surface during the drying process. The mesh is lapped and tied together with wire so that movement of the reinforcement in constrained for the duration of the installation and the concrete pour. In some designs reinforcing bar is combined with the mesh to provide additional design strength where required.
In larger span and thicker suspended slabs, reinforcement bars are generally used in the design and installation, along with reinforcement cages in integrated concrete beams where additional span capacity is required. The reinforcement bar is designed in layers, two layers adjacent the bottom portion of the slab and two layers adjacent the top portion of the slab. Every layer has bars installed at a regular spacing running parallel to each other, the next installed layer has the bars placed at 90 degrees to the ones below and spaced equidistant from each other to the design requirements. The same applies to the construction of concrete walls using formwork shutters. In beams and columns, the main reinforcement bars laid in the direction of the column or beam usually have stirrups and ligatures tied around them, to keep the main bars in a particular arrangement for the design, as well as to transfer shear. The assembly of the primary bars and ligatures/stirrups can be termed a wire cage, and in many cases are manufactured off site using welding, and shipped to site and installed as a complete unit. In a lot of cases, the beam cages are assembled on site and take considerable time to do so, since the ligatures need to be threaded over the reinforcement bars and tied together at regular spacing along the bars and tied to the bar as well. The assembled cage can be heavy and poses considerable difficulty in chairing from the formwork to create sufficient concrete cover during the pour.
In suspended slabs, the bar installation process requires the bar spacing to be set to the engineering requirements, and the spacing is generally the same for each layer of steel although it can vary from one layer to the other. The installation process is carried out by manual labour and the occupation is termed steel fixing, and the tradesman, a Steel Fixer. The steel fixer will mark the spacing on top of the formwork for the first layer of bar and place it in accordance with the markings. The bottom steel at this point is not restrained from moving from side to side until a bar from the next layer is installed at 90 degrees to them. The bar from the first layer is then tied to the bar above while maintaining the required spacing. A difficulty in this process is the need to lift the bottom steel bar from the formwork to allow the wire tie to slide underneath and be tied to the cross bar. It becomes increasingly difficult to lift the bars from the bottom layer as the steel tying progresses because more of the previously connected bars add to the lift weight combined with careful foot placement to avoid stepping on the pre tied steel. In many cases, strip chairs are employed to support the bottom layers of steel during the tying process, which can alleviate but does not fix the problem entirely, as manual labour is still required to install and place the chairs.
After the placement and tying the first two layers, the mesh assembly is lifted and placed upon bar chairs. Sometimes this is done during the tying process. The more common bar chairs are made from wire and have four legs, hence the name chair. These are spaced at approx. 800 mm apart in a grid arrangement and are installed so the bottom two layers of mesh have adequate concrete cover when the slab is poured. Other spacing may be suitable for other sizes of steel which can support larger spans, and in cases where the compression capacity of the chair and the formwork it rests on allows it.
The process is repeated for the top two layers of reinforcement steel by building the mesh arrangement on top of the chaired bottom layer. The steel tying becomes easier since the steel is supported by the mesh below in discrete locations, rather than continuously on top of the formwork as in the first layer of the bottom steel bars. When the steel bars are tied, the top two layers are chaired to the design height by using a higher bar chair and is secured in place. Sometimes the higher bar chairs are installed first and the bars are progressively tied at the appropriate height, however managing and controlling the tying process is much more difficult. The design height of the top steel is determined by the concrete cover that is required for the concrete strength employed and environmental conditions. A difficulty in this process is in lifting the upper steel mesh while supporting the feet on the lower steel mesh below without the feet impeding the upper mesh being lifted. As the mesh opening size decreases, the difficulty in lifting the tied steel increases and as the steel fixer foot size (and subsequent boot size) increases then the difficulty in tying and lifting the upper mesh around the boots increases. Another problem associated with using taller wire chairs is they may collapse under the weight of the mesh load combined with concreters walking on the mesh during the concrete pour, reducing the strength of the reinforced concrete slab in that area if is not corrected. The tying process is slowed further by the need to continuously check the spacing of the bars and to maintain the engineered design of the amount of steel cross sectional area per meter of span of slab at right angles to the direction of the bars.
The chairing requirements are also applied to pre made mesh, the bottom layer is chaired above the formwork, and the top layer is supported by another chair installed afterwards.
Reinforcement bar is supplied to site in deformed and straight lengths ready for installation by the steel fixers. All the reinforcement bars and deformed bars are located in the slab by a numbering system which is part of the steel scheduling system. None of the bars carries any particular information on them except when they are bundled together and tied together, the attached tag lets the steel fixer know where these bars are to be installed on the reinforcement schedule layout.
Bar chairs and other ancillary fixing equipment is supplied separately and are called accessories.
The manufacturing systems generally delivers bars to particular shapes and to particular lengths and has no ability to build in smart information with the manufactured component and bring it to the site so that the steel fixers may know, without reference to drawings at what spacing and how to install it.
The present invention aims to overcome or alleviate the above disadvantages by providing a more cost effective and user friendly way of providing chair support in the early stages of reinforcement bar installation and provide a simpler method of lifting and tying the steel.
The present invention seeks to overcome or substantially ameliorate at least some of the deficiencies of the prior art, or to at least provide an alternative.
It is to be understood that, if any prior art information is referred to herein, such reference does not constitute an admission that the information forms part of the common general knowledge in the art, in Australia or any other country.